Computer communications networks for allowing computers to communicate data to and from other computers have become common. For example, a user of a first computer can send and receive files and real-time data to and from a second computer. A local-area network (LAN) is a computer communications network which provides computer communications among a plurality of computers situated within a common locale. For example, a LAN is typically used to interconnect personal computers or workstations within an office or school building, or to interconnect computers situated in several buildings of a campus or office park. The computers connected to the LAN typically communicate among one another, and usually also communicate with one or more centralized or specialized computers, such as a host computer, with an output device, such as a printer, and with a mass data storage device, such as a file server.
A computer communications network, such as a LAN, employs a transmission medium to communicate data signals among the plurality of data devices in the network. Usually, the transmission medium is a network of wires. Wires can be cumbersome in that they can present routing problems, occupy space, require installation time, and inhibit the mobility of the computers connected to the network.
To overcome the problems associated with using a system of wires as the transmission medium, a plurality of radio transceivers can be used to communicate radio signals for carrying data messages among the computers in the computer communications network. Use of radio transceivers has gained little acceptance so far due to low data transmission rates and/or unreliability. Typically, if the data transmission rate is lowered, the reliability can be improved. Alternatively, high data transmission rates can be achieved, albeit with reduced reliability.
The principle barrier to high data rate communications between computers in a wireless local-area network is an interference phenomenon called "multipath". A radio signal commonly traverses many paths as it travels towards a receiver. Multiple propagation paths can be caused by reflections from surfaces in the environment, for example. Some of these paths are longer than others. Therefore, since each version of the signal travels at the same speed, some versions of the signal will arrive after other versions of the signal. Sometimes the delayed signals will interfere with more prompt signals as the delayed signals arrive at the receiver, causing signal degradation.
Multipath time-delay spread is the time that elapses between the moment that the earliest version of a transmitted signal arrives at a receiver, and the moment that the latest version of the signal arrives at the receiver.
To understand multipath effects and the instant invention, it is helpful to discuss the term "symbol". One or more symbols can be combined to form a message that conveys meaning. Each symbol must be uniquely recognizable, and is selected from a set of possible symbols, referred to as a symbol alphabet. The number of symbols in the symbol alphabet is referred to as the "order" of the symbol alphabet. For example, the letters "a", "b", and "c" are symbols from the English alphabet, where the order of the English alphabet is 26. The numbers "0" and "1" are symbols of the binary number system, which is of order 2.
It is possible to represent a sequence of symbols from a first alphabet with a symbol from a second alphabet, such as representing the binary symbol sequence "101" by the symbol "a". This binary symbol sequence consists of three binary symbols. Since each binary symbol can be either one of two possible symbols, in a sequence of three binary symbols, there are eight possible unique binary symbol sequences. Thus, an alphabet of order eight is required to represent the eight possible unique binary symbol sequences of three symbols each. In general, an alphabet of order M=2.sup.N is required to represent the M=2.sup.N possible unique binary symbol sequences of N symbols each.
Just as binary signalling can be referred to as 2-ary signalling, a signalling system that represents three-element binary symbol sequences using a symbol alphabet of order eight is referred to as 8-ary signalling. In the terminology of communications system design, an 8-ary symbolic representation is said to represent each symbol using "3 bits per symbol".
In general, a signaling system that represents an N-element binary symbol sequence using a symbol alphabet of order M=2.sup.N is referred to as M-ary signalling. In M-ary signalling, the equivalent binary data rate R is the symbol rate S multiplied by the number of bits per symbol N, i.e., R=S*N. The number of bits per symbol N is log.sub.2 M. Thus, for 8-ary signalling, N=3, and therefore the equivalent binary data rate is three times the symbol rate (assuming no error correction coding and no overhead bits).
In binary signalling, the equivalent binary data rate is equal to the symbol rate, i.e, R=S, because when M=2, the number of bits per symbol N is one. Consequently, "bit" and "symbol" are often used interchangeably in discussions of binary signalling.
In radio communications, a transmitter includes a modulator that provides a transmitted signal representative of information presented to the modulator. Conversely, a receiver includes a demodulator that receives the transmitted signal and ideally provides the original information represented by the transmitted signal. Commonly, the information presented to the modulator includes a plurality of symbols, where each symbol is selected from a finite set of symbols. For each symbol presented to the modulator, the modulator generates a corresponding symbol waveform selected from a set of discrete symbol waveforms, the symbol waveform then being transmitted over a communications channel to be received by at least one receiver.
Each symbol waveform that is transmitted is subject to distortion and noise, thereby making each received symbol waveform differ from the corresponding original transmitted symbol waveform, and become more similar to other symbol waveforms that were not actually transmitted. Consequently, it is necessary to decide which symbol of the discrete set of known symbols was most likely transmitted. This decision is performed in the demodulator of the receiver, the output of the demodulator being a sequence of symbols, selected from the known set of symbols, that represents the best estimation of the transmitted symbol sequence.
To decide which symbol sequence has been transmitted, for each transmitted symbol, the demodulator processes the corresponding received symbol waveform for a period of time called a coherent integration interval. It is essential that each coherent integration interval be coincident with each received symbol waveform, thereby providing correct synchronization. In the absence of correct synchronization, the symbol content of the received waveform will be misinterpreted.
To further clarify the concept of multipath interference, consider the case of a message transmitted as a binary data modulation waveform, wherein each message symbol consists of a single bit. When the multipath time-delay spread is longer than the duration of a symbol waveform, symbol waveforms of the first version of the received signal overlap non-corresponding symbol waveforms of the excessively delayed versions of the received signal. This phenomena is called intersymbol interference (ISI).
For example, in a typical indoor or campus radio network environment, the time-delay spread can be greater than 500 nanoseconds (ns). Since in binary data modulation, data rate is the multiplicative inverse (reciprocal) of symbol duration, a time delay spread of 500 ns implies that data rates even much less than two million bits per second (Mbps) will result in significant data errors due to intersymbol interference.
In addition to intersymbol interference, some multipath reflections may exhibit time-delay spreads that are less than the duration of a symbol waveform. This form of multipath interference is referred to as intrasymbol interference, and such interference can cause a significant degradation in the amplitude of the total received signal.
In intrasymbol interference, the multipath time-delay spread is shorter than the duration of a symbol waveform. Thus, symbol waveforms of the first received signal version overlap non-corresponding portions of corresponding symbol waveforms of the delayed versions of the received signal. Consequently, reflected signals of significant amplitude will cause periodic amplitude nulls in the frequency spectrum of the total received signal due to coherent cancellation at particular frequencies. The bandwidth of the amplitude nulls is inversely proportional to the delay of the corresponding signal that is causing the interference. This phenomenon is known as "frequency selective fading", and it substantially impairs the reliability of communication between a transmitter and a receiver.
Overcoming frequency selective fading is commonly accomplished using diversity methods. These methods include spatial diversity, polarization diversity, and frequency diversity. Spatial and polarization diversity require at least two receivers, each having a separate receive antenna, such that the frequency selectivity pattern is different for each antenna.
Frequency diversity receivers can share a single broadband receive antenna, but the transmitted signal is duplicated and is transmitted on at least two carrier signals that are separated by a frequency bandwidth that is larger than the width of a frequency null. A frequency diversity receiver unit consists of multiple receivers, each tuned to a different carrier frequency. The receiver outputs of either method fade independently, and are combined in one of several known ways to take advantage of this. Since this method employs an independent receiver for each diversity channel used, it can be quite costly to implement.
There are known methods for reducing intersymbol interference due to multipath effects while preserving high data rates. A first method employs highly directional line-of-sight microwave links with high antenna gain, since signals having the longest delays often arrive at angles far from the central axis of the microwave antenna. One problem with this method is that to obtain high antenna gain, the antennas must be large, mounted on fixed platforms, and must be carefully pointed. Such antennas are therefore complicated and expensive to install and move. Large antennas are particularly unsuitable for short range indoor or campus environments.
A second method for reducing intersymbol interference due to multipath effects while preserving high data rates is to use echo canceling techniques implemented using adaptive filters. However, the expense and computational requirements of adaptive filters is prohibitive at the high data rates required in the highly dynamic environment of radio communications.
A third method is to channelize the transmitted waveform into multiple channels, each channel being of different carrier frequency and of lower bandwidth (therefore using longer symbol durations) than the single-channel transmitted waveform. Each channel is then received independently. This approach is excessively costly because one independent receiver per channel is required.
A fourth and less conventional approach is to use M-ary orthogonal signalling, with symbols that are log.sub.2 M times as long as the binary symbols would be. According to the property of orthogonality, the waveform that represents each symbol has no projection on the respective waveform of any other symbol of the symbol alphabet from which the symbols of the message are selected. Consequently, each symbol in the alphabet is more easily distinguished from other symbols in the alphabet than without the orthogonality property.
If the temporal symbol duration of the orthogonal signal is made much longer than the multipath time delay spread, the effect of the multipath can be reduced. For example, one of many approaches includes the use of M-ary frequency shift keying (MFSK) modulation to encode the high-order symbol alphabet into one of M frequencies. Orthogonal signaling would still require a diversity receiver to overcome intrasymbol interference. Furthermore, orthogonal signalling requires excessive bandwidth to implement as compared with a conventional communications channel, and is therefore typically prohibited by government regulation.
All of these approaches for reducing intersymbol interference due to multipath effects while preserving high data rates must, in general, also include means for diversity reception to reduce the intrasymbol interference, and consequently must employ duplicate receivers.
Direct-sequence spread spectrum (DSSS) modulation is a multiplicative modulation technique that can be used for resolving and discriminating against multipath interference. A common but unsatisfactory approach to mitigating multipath effects is to employ direct-sequence spread spectrum modulation in conjunction with binary data modulation, where the direct-sequence spreading function of the DSSS modulation is a pseudonoise (PN) waveform. This approach is unsatisfactory because it cannot provide sufficiently high data rates to support LAN throughput requirements when sufficient processing gain is used for overcoming multipath effects.
The processing gain of a binary-data-modulated spread spectrum waveform is the ratio of the spreading bandwidth of the DSSS modulation to the data bandwidth. The spreading bandwidth is often limited due to constraints imposed by government regulation or by shortcomings of signal processing technology. Lowering the binary data rate increases processing gain and consequently robustness, but sacrifices rate of data throughput.
The ability to reduce both intersymbol and intrasymbol interference due to multipath effects depends on the processing gain of the spread spectrum waveform and receiver, whereas the ability to resolve adjacent paths is a function only of the spreading bandwidth, not of the symbol rate.
It is known to use Walsh-function waveforms to implement code division multiple access (CDMA). CDMA is used to improve the channel capacity of a spread spectrum system where multiple transmitters share the same frequency spectrum. Walsh-function modulation is used to provide separable signals. It is difficult to ensure this separability because of limited processing gain, and hence precise transmitter power regulation is usually required. Further improvements in gain would be desirable.
Gilhousen, U.S. Pat. No. 5,103,459, specifically teaches a cellular telephone system that employs spread spectrum encoding to discriminate among the signals of multiple users. This capability illustrates a well-known CDMA application of spread spectrum signalling. Reduction of multipath interference is not addressed. In the forward channel, Walsh-function signalling is used for improved CDMA performance, not for data modulation. Furthermore, Walsh-function signalling is not used to increase the CDMA processing gain by extending the symbol duration, but only to provide a better CDMA waveform than pseudo-noise DSSS would alone provide, because the treatment is such that the orthogonality property occurs between multiple users sharing the same frequency band, and not between data symbols. Although Gilhousen '459 discusses the use of Walsh-function data modulation in the reverse channel, Gilhousen '459 clearly states that the purpose of the Walsh-function signalling is to obtain good Gaussian noise performance in a Rayleigh fading multipath channel. Consequently, use of a modulation, such as binary phase shift keying, that requires a coherent phase reference signal for demodulation is precluded. Gilhousen '459 states that differential phase shift keying will not operate well in a Rayleigh fading multipath environment, and some means of orthogonal signalling is required to overcome the lack of a phase reference. Moreover, since the multipath channel discussed in Gilhousen '459 is Rayleigh fading, Gilhousen '459 does not resolve and discriminate against multipath interference. Further, the use in Gilhousen '459 of Walsh-function signalling for data modulation is independent of the use therein of spread spectrum encoding. Gilhousen '459 explicitly states that binary orthogonal signalling also works, since a coherent phase reference would not be required. The receiver described in Gilhousen '459 requires that the entire forward and backward channel be utilized to time-synchronize the mobile units. In fact, a satellite-based timing system is required to keep time aligned between cells. Therefore, the system disclosed by Gilhousen '459 is clearly a time-synchronous CDMA cellular telephone communications system, and is not intended for, or useable as, a high data-rate radio-frequency inter-computer communications system.
Kerr, U.S. Pat. Nos. 4,635,221 and 5,001,723, describes a system that utilizes the bandwidth available in a surface-acoustic-wave convolver, which generally has a much higher processing bandwidth than the bandwidth available for signal transmission. A received signal is multiplexed onto several carrier frequencies, and each is processed independently in the convolver. The convolver is used to simultaneously compare the received signal to M orthogonal reference waveforms, composed of Walsh function and PN-DSSS waveforms. The '723 patent describes a variation that uses orthogonal sinusoids instead of Walsh functions, as taught by the '221 patent. The scope of the teachings of these patents is narrow in that they specifically address a method of demodulating a plurality of signals using a convolver, and do not disclose any means for implementing a high-data-rate wireless local-area-network suited for use in a multipath environment.
Groth, U.S. Pat. No. 4,494,238, discloses use of pseudo-noise direct-sequence spread spectrum across multiple, non-contiguous carrier frequencies that are coherently processed at a receiver. Walsh functions are used in this system to generate signals within the receiver, such signals being used to perform phase computations, but not for signaling over a communications channel corrupted by multipath interference.
McRae et al., U.S. Pat. No. 4,872,182, provides a method for determining a useful frequency band for operating a high frequency radio communications network. Each receiver is identified by its pseudonoise direct-sequence spread spectrum reference code, which implies that spread spectrum encoding is used for CDMA purposes, even though the term "CDMA" is not explicitly mentioned. Walsh-function modulation is used to specify control information for scanning available frequency bands until a useful frequency band is found.